ISDN is a relatively newly developed and emerging field of telecommunications which integrates computer and communications technologies to provide, worldwide, a common, all-digital network. This is based, in part, on standardizing the structure of digital protocols developed by the International Telegraph and Telephone Consultative Committee (CCITT). Despite the implementation of multiple networks within national boundaries, from a user's point of view there is a single, uniformly accessible, worldwide network capable of handling a broad range of telephone, data and other conventional and enhanced services.
A complete description of the architecture of ISDN is beyond the scope of this specification. For details, and for an extensive bibliography of references of ISDN, see Stallings, ISDN, An Introduction, MacMillan Publishing Company, 1989.
An ISDN is structured by architecture closely following the OSI Seven Layer Reference Model. Within the framework of ISDN, the network provides services and the user accesses the services through the user-network interface. A "channel" represents a specified portion of the information carrying capacity of an interface. Channels are classified by two types, Basic Rate ISDN (BRI) and Primary Rate ISDN (PRI). BRI delivers two B-channels, each having a capacity of 64 Kbps, capable of transmitting voice and data simultaneously. A 16Kbps D-channel transmits call control messages and user packet data. PRI provides twenty three B-channels of 64 Kbps capacity each for carrying voice, circuit switched data or packet data. The D-channel is a 64Kbps signaling channel. The B and D channels are logically multiplexed together at Layer 1 of the OSI Reference Model.
With reference to FIG. 1, the conventional ISDN interfaces are depicted. At the customer premises, an "intelligent" device, such as a digital PBX terminal controller or Local Area Network (LAN), can be connected to an ISDN terminal TE, such as a voice or data terminal, which is connected to a Network Termination (NT1). Non-ISDN terminals TE may be connected to a Network Termination (NT2) [over the RS-232 Interface] and a Terminal Adapter TA. The NT2 in turn is connected over an "S/T-Interface", which is a four-wire bus, to a termination NT1 that performs functions such as signal conversion and maintenance of the electrical characteristics of the loop.
At the local loop, a two-wire bus, termed the "U-Interface", or "Loop", interconnects NT1 and a Loop Termination (LT) at the central office. Finally, the "U-Interface" is a bus between the local loop at the carrier end and exchange switching equipment. Details of this architecture are provided in ISDN: An Overview, Data Pro Research, Concepts & Technologies, MT 20-365; pp 101-110, published by McGraw Hill Incorporated (December 1988).
ISDN can be used to service the needs both of public data telephony and private networks. In general, access to public telephone is performed as shown in FIG. 2(a). An initiating host H first issues dialing commands to the public network to set up a connection with the destination host H, and then uses the connection to communicate with the destination host. The connection either is circuit switched or packet switched. Data communication in private networks incorporates routers, generically called Interface Message Processors (IMPs), which may also be used as hosts. In FIG. 3(a), a host A communicating with host C will take an appropriate route through an IMP interface.
To establish communications between a host and the ISDN, a gateway operating at the upper layers of the OSI reference model accomplishes gateway functions, translating protocols used on dissimilar networks. An ISDN gateway, designated by GW in FIG. 1, will operate entirely in the digital domain, carrying out all necessary protocol conversion between the host network and ISDN.
Copending application Ser. No. 08/094,144 to Gagliardi et al. entitled "ISDN Interfacing of Personal Computers", filed on even date herewith and assigned to the assignee of the present invention, describes a gateway interconnecting personal computers and the ISDN in a manner shown symbolically in FIG. 2(b). The gateways GW in FIG. 2(b) carry out all necessary protocol conversion to enable the user to transparently access the ISDN.
A related commonly assigned application Ser. No. 08/094,114 to Gagliardi et al., entitled "Method of and System for Accessing Distributed Resources on ISDN", filed on even date herewith, is directed to implementation of the gateway GW to enable computers interconnected to each other and to the ISDN to share resources by issuing commands in operating system commands. One computer can access a storage disk of another computer distributed anywhere in a virtual network established on the ISDN. Each disk throughout the networks has a unique name by which it is accessed. The configuration of the network and number of computers distributed on it are transparent to each user.
In FIG. 3(b), support for existing private networks is transparently accomplished by providing an ISDN gateway GW of a type described in copending application Ser. No. 07/890,588 to Gagliardi et al., entitled "ISDN Interfacing of Personal Computers", which emulates the current interface between a host in the private network and the IMPs that comprise the network, or by emulating the interface to a fixed point-to-point communication link, such as a leased T1 line. Users simply replace the connections to IMPs to connect to an ISDN gateway. FIG. 3(c) depicts a variation replacing the connection between an IMP or host and the communication line with that to an ISDN gateway; the IMPs and hosts in this configuration are equivalent.
Copending application Ser. No. 08/094,143 to Gagliardi et al., entitled "ISDN Interfacing of Local Area Networks", filed on even date herewith and assigned to the common assignee, describes a gateway supporting interconnection among hosts on the ISDN to form a "virtual". A local area network, or LAN, provides a cluster of interconnected hosts (computers), or nodes, on a medium. Each node can communicate with every other node; the network requires no central node or computer. Base band LAN systems, such as Ethernet, impress data signals directly on the network medium, whereas broad band systems modulate a very high frequency carrier with the data signal before impressing it on the medium. The architecture of each LAN conforms with the OSI reference model. Other physical media standards include Token Ring and Token Bus. Several different network operating systems have been adopted by the industry.
FIG. 4 depicts utilization of one type of gateway GW described in copending application entitled "ISDN Interfacing of Local Area Networks" for interconnecting a number of hosts to form a virtual LAN. A variation, shown in FIG. 5, provides ISDN gateways GW to interconnect to LANs of a common type or of different types using a common naming convention. In FIG. 6, a remote host H is connected to a pair of hosts residing on a LAN through first and second gateways GW at the host and LAN branches of the ISDN. The hostside gateway GW1 is of a type described in the copending application entitled "ISDN Interfacing of Personal Computers". The LAN side gateway GW2 in the Figure is of a type described in the copending application entitled "ISDN Interfacing of Local Area Networks".
Interconnection of hosts with a LAN facility or forming a virtual LAN is based on our discovery that point-to-point communications on the ISDN is of the same order of magnitude as that of a high speed local area network. This discovery is counterintuitive, as the nominal throughput of an ISDN is 64 Kbps whereas that of a local area network is in the range of between 10 and 16 Mbps. Accordingly, it would appear that interconnecting a LAN to an ISDN would not be feasible from a performance point of view.
One factor underlying this surprising "natural" performance adaptation between Mbps LANs and Kbps ISDN is due to a number of factors. One factor is the very significant difference between actual point-to-point LAN throughput an LAN rated bandwidth. For example, the throughput of a 10 Mbps Ethernet-based LAN typically ranges from 56 to 79 Kbps, less than the capacity of a single ISDN B- channel. Our observations indicate the surprising result that while the very large bandwidth can accommodate many simultaneous point-to-point sessions, it is fully compatible for the ISDN to provide long distance interlinking for such sessions.
In general, utilization of bandwidth between hosts on any network is very inefficient. In a file transfer operation, a large amount of data typically is transferred from one computer to another in a single continuous operation. This can occur when a local computer requires a local copy of remote data or updates a remote centralized data bank with fresh local data. Data transfer may be in the form of a data call lasting at least several hours, with bursts of from 100,000 to 1,000,000 bytes being transferred every ten to twenty minutes. Such an application utilizes the communication link with a duty cycle which is in the range of 30 to 2 seconds, assuming that the operating system of the LAN is capable of transferring 30 to 50 Kbps. In this example, file transfer results in very low channel utilization, on the order from less than one percent to slightly greater than five percent. Channel utilization for data base query and transaction processing similarly is on the order of much less than one percent.